Circuit measures battery capacity

Batteries and energy cells lose
their capacity as they age. If a cell
or battery’s capacity is too low, your
equipment may also soon stop working.
You can use the circuit in Figure 1 to
measure a battery’s discharge time. The
circuit uses an electromechanical clock
and a DVM (digital voltmeter). The cell
should be fully charged before testing.
The circuit discharges the cell at a fixed
current and measures the time it takes to
discharge the cell from 100 to 0%.

For example, if a manufacturer rates
a cell’s capacity and you discharge the
cell at a constant current equal to 0.1
times the capacity, the cell should
take about 10 hours to discharge from
full to empty. Manufacturers of NiCd
(nickel-cadmium) or NiMH (nickel-metal-hydride) cells rate the end of the
discharge voltage at 1V. At that point,
the cell is using 0% of its capacity, is
flat, and requires charging for further
operation. If this procedure takes less
than 10 hours, the cell’s capacity is less
than what the cell manufacturer rates.

Before testing the cell, charge it to
full capacity using your charger. Apply
12V dc to the circuit and use potentiometer
P2 to set a voltage of 1V at Pin
6 of IC1B. Set the clock to 12:00. An
AA-size, 1.5V cell powers the clock
through relay switch S3.

When you press the momentary
pushbutton switch, S1, the tested cell
starts to discharge through transistor Q1
and resistor R1. Set the discharge current
using potentiometer P1. Op amp
IC1A keeps the voltage across resistor R1
constant, thus providing stable cell-discharge
current. Set the DVM to measure
the dc voltage and measure the voltage
across R1. The display shows discharge
current in amperes. For example, 0.25V
corresponds to 0.25A. Because the initial
cell voltage is higher than 1V, Pin 7 of op
amp IC1B is high, transistor Q2 is on, and
the DPST (double-pole/single-throw)
relay coil is active. Relay-contact switch
S2 closes and bypasses the start pushbutton
switch, S1, which keeps the discharge
process active. Closed relay-contact
switch S3 lets the clock keep time.

When the cell’s voltage is equal to
the end-of-discharge value, 1V, IC1B’s
output goes low and deactivates the
relay coil, halting the discharge process.
The clock also stops. To get the cell’s
capacity, multiply the set dischargecurrent
value by the elapsed time. If
the discharge-current value is small and
the time necessary for the discharge of
a cell is longer than 12 hours, you must
check this value every 12 hours after
you start the test and keep in mind laps
of one to 12 hours.

This circuit also lets you estimate the
self-discharge rate of the cell or battery
you use. Charge your cell to 100% of
its capacity and measure cell capacity according to this procedure. Charge
your cell again, store it for a month, and
then measure the cell capacity again.
The difference between the two values
is the monthly self-discharge rate.

If you arrange the cells in a stack,
you should provide a reference voltage
that’s higher than the battery’s end-ofdischarge
voltage. If the battery voltage
is higher than 12V, use a higher-voltage
value to power the circuit. Furthermore,
the reference voltage value should be
higher than the battery’s
end-of-discharge value.
Specifications of the discharge
path comprising
transistor Q1 and resistor R1
should fit higher dischargecurrent
requirements.

The circuit works with
cells or batteries of any
chemistry, including NiCd,
NiMH, lead acid, and lithium-
ion. You can also use
this circuit to measure the
real capacity of nonrechargeable
cells, such as AA alkaline
cells. In that case, the
discharged cell’s voltage
should be equal to the lowest
power-supply voltage of
your device. A cell that has
passed the test is not suitable
for further use, but you
can use its capacity information
to estimate the capacity
of the batteries of the same
type and manufacturer.